chapter
4
Energy
Expenditure
and Fatigue
Measuring Energy Costs of
Exercise
Direct calorimetry measures the body’s heat
production to calculate energy expenditure.
Indirect calorimetry calculates energy expenditure
from the respiratory exchange ratio (RER) of CO2
and O2.
A Direct Calorimeter for Human
Use
MEASURING RESPIRATORY
GAS EXCHANGE
Respiratory Exchange Ratio
•
•
•
•
•
.
The ratio between
CO2 released (VCO2) and oxygen
.
consumed (VO2).
.
.
RER = VCO2/VO2.
C6 H12 O6 + 6O2  6CO2 + 6H2O + 38 ATPs
RER for CHO is 1.0; RER for Fat is .70
The RER value at rest is usually 0.78 to 0.80.
Estimating Anaerobic Effort
• Examine excess postexercise oxygen consumption
(EPOC)—the mismatch between O2 consumption and
energy requirements.
• Note lactate accumulation in muscles.
Oxygen Requirement During Exercise
and Recovery
Factors Responsible for EPOC
• Rebuilding depleted ATP supplies
• Clearing lactate produced by anaerobic metabolism
• Replenishing O2 supplies borrowed from hemoglobin
and myoglobin
• Removing CO2 that has accumulated in body tissues
• Feeding increased metabolic and respiratory rates due
to increased body temperature
Lactate Threshold
• It is the point at which blood lactate begins to
accumulate above resting levels during exercise of
increasing intensity.
• Sudden increase in blood lactate with increasing effort
can be the result of an increase in the production of
lactate or a decrease in the removal of lactate from the
blood.
• It can indicate potential for endurance exercise; lactate
formation contributes to fatigue.
Relationship Between Exercise Intensity
and Blood Lactate Concentration
Did You Know . . . ?
Lactate threshold
. (LT), when expressed as a
percentage of VO2max, is one of the best determinants
of an athlete’s pace in endurance events such as
running and cycling. While untrained people
typically
.
have LT around 50% to 60% of their VO2max, elite
athletes
may not reach LT until around 70% or 80%
.
VO2max.
Key Points
Measuring Energy Use During Exercise
• Direct calorimetry measures the heat produced by
the body, while indirect calorimetry measures the
ratio of O2 consumption to CO2 production.
• Comparing the RER value to standard values
determines what fuels are being oxidized and
calculates the energy expended per liter of O2
consumed.
• Tracking ingested or injected isotopes in the body
can also be used to calculate CO2 production and
caloric expenditure.
(continued)
Key Points (continued)
Measuring Energy Use During Exercise
• Excess postexercise oxygen consumption (EPOC)
is the elevation of O2 consumption above resting
levels after exercise; it is caused by a combination
of several factors.
• Lactate threshold is the point at which blood lactate
above resting levels begins to accumulate during
exercise.
• Individuals with higher lactate thresholds
. or OBLA
values, expressed as a percentage of VO2max, are
capable of the best endurance performance.
Metabolic Rate
• It is the rate at which the body expends energy at rest
and during exercise.
• It’s measured as whole-body oxygen consumption and
its caloric equivalent.
• Basal metabolic rate (BMR) is the minimum energy
required for essential physiological function (1,200 and
2,400 kcal).
• Resting metabolic rate (RMR) is the minimum energy
required for normal daily activity (1,800 to 3,000 kcal).
Factors Affecting BMR
•
•
•
•
•
•
The more fat mass, the lower the BMR.
The more body surface area, the higher the BMR.
BMR gradually decreases with increasing age.
BMR increases with increasing body temperature.
The more stress, the higher the BMR.
The higher the levels of thyroxine and epinephrine,
the higher the BMR.
.
Maximal Oxygen Uptake (VO2max)
• Upper limit of a person’s ability to increase oxygen
uptake.
• Good indicator of cardiorespiratory endurance and
aerobic fitness.
• Can differ according to sex, body size, age, and, to some
degree, level of training.
• Expressed relative to body weight in ml of O2 consumed
per kg body weight per min (ml · kg-1 · min-1). College
aged females and males approx. 38, 44, respectively
• Elite athletes are > 70
Determining Success in Endurance
Performance
•
•
•
•
.
High maximal oxygen uptake capacity (VO2max)
High lactate threshold
High economy of effort
High percentage of slow-twitch muscle fibers
Factors Influencing Energy Costs
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•
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Type of activity
Activity level
Age
Sex
Size, weight, and body composition
Intensity of the activity
Duration of the activity
Efficiency of movement
Key Points
Energy Expenditure at Rest and During Exercise
• The basal metabolic rate (BMR) is the minimum
amount of energy required by the body for basic
physiological functions.
• The resting metabolic rate is the BMR as well as the
typical daily caloric expenditure.
• Your metabolism increases with increased exercise
intensity.
(continued)
Key Points (continued)
Energy Expenditure at Rest and During Exercise
• Oxygen consumption
increases during exercise to its
.
upper limit (VO2max).
• Performance improvements often mean that an
individual can perform for. longer periods at a higher
percentage of his or her VO2max.
• Performance capacity can be improved by increasing
economy of effort.
Causes of Fatigue
• Phosphocreatine (PCr) depletion
• Glycogen depletion (especially in activities lasting
longer than 30 minutes)
• Accumulation of lactic acid and H+ (especially in events
shorter than 30 minutes)
• Neuromuscular fatigue
• Stress
Decline in Muscle Glycogen
Adapted, by permission, from D.L. Costill, 1986, Inside running: Basics of sports physiology (Indianapolis:
Benchmark Press). Copyright 1986 Cooper Publishing Group, Carmel, IN.
Time to Exhaustion
Adapted, by permission, from S.D.R. Galloway and R.J. Maughan, 1997, "Effects of ambient temperature on
the capacity to perform prolonged cycle exercise in man," Medicine and Science in Sports and Exercise 29:
1240-1249.
Metabolic By-Products and Fatigue
• Short-duration activities depend on anaerobic
glycolysis and produce lactate and H+.
• Cells buffer H+ with bicarbonate (HCO3–) to keep cell
pH between 6.4 and 7.1.
• Intercellular pH lower than 6.9, however, slows
glycolysis and ATP production.
• When pH reaches 6.4, H+ levels stop any further
glycolysis and result in exhaustion.
Key Points
Causes of Fatigue
• Fatigue may result from a depletion of PCr or
glycogen, which then impairs ATP production.
• The H+ generated by lactic acid causes fatigue in
that it decreases muscle pH and impairs the cellular
processes of energy production and muscle
contraction.
• Failure of neural transmission may cause some
fatigue.
• The central nervous system may also perceive
fatigue as a protective mechanism.